Read more: 鈥�10 biggest puzzles of human evolution鈥�
NOBODY would mistake a human for a chimpanzee, yet we share more DNA than mice and rats do. How can that be? Advances in genomics are starting to unravel the mystery.
Line up the genomes of humans and chimps side by side and they differ by little more than 1 per cent. That may not seem like much, but it equates to more than 30 million point mutations. Around 80 per cent of our 30,000 genes are affected, and although most have just one or two changes (), these can have dramatic effects. The protein made by the human gene FOXP2, which helps us to speak, differs from its chimp counterpart by just two amino acids, for example. And small changes in the microcephalin and ASPM genes may underlie big differences in brain size between humans and chimps.
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But protein evolution is only part of what makes us human. Also critical are changes in gene regulation 鈥� when and where genes are expressed during development 鈥� says James Noonan of Yale University. Mutations in key developmental genes are likely to be fatal. But, he says: 鈥淎ltering the expression of a gene in a single tissue or at a single time can more easily lead to an innovation that is not lethal.鈥� Noonan鈥檚 lab is one of many that are busy comparing gene expression in tissues such as the brain to home in on the key regulatory difference between chimps and humans, most of which have still to be uncovered.
Then there鈥檚 gene duplication. This can give rise to families of genes that diversify and take on new functions, says Evan Eichler at the University of Washington in Seattle. His lab has identified uniquely human gene families that affect many aspects of our biology, from the immune system to brain development. He suspects that gene duplication has contributed to the evolution of novel cognitive capacities in humans, but at a cost: greater susceptibility to neurological disorders.
Copying errors mean whole chunks of DNA have been accidentally deleted. Other chunks find themselves in new locations when mobile genetic elements jump around the genome or viruses integrate themselves into our DNA. The human genome contains more than 26,000 of these so-called INDELs, many linked with differences in gene expression between humans and chimps ().
鈥�98.5% DNA shared by chimps and humans鈥�
Even a complete catalogue of genetic differences will not solve the mystery. Much of what makes us human is cultural, passed from generation to generation by learning, says Ajit Varki at the University of California, San Diego. What鈥檚 more, he says, The co-evolution of genes and culture is a major force in human evolution, famously leaving the descendents of dairy farmers able to digest milk protein, for example. To crack the mystery of human uniqueness we need to know how genomes build bodies and brains, how brains create culture, and how culture eventually feeds back to alter the genome. It remains a distant goal.
When this article was first posted, it incorrectly stated that Evan Eichler was at Washington State University
